1. Field of the Invention
The invention relates to a method and apparatus for transmitting signals from the bottom of a well bore to the surface by means of mud pressure pulses generated within the hydraulic flow in the drill string.
2. Description of the Prior Art
The desirability of telemetry systems for transmitting downhole information while drilling through the mud column in the drill string has long been recognized. Equipment and procedures for control and monitoring of mud flow parameters are widespread and readily understood in the industry. Therefore, the use of propagating mud pulses through the drill string for the purpose of communicating information from the down hole location while drilling to the well's surface is also widely used and understood.
Prior art mud pulsing devices are generally classified in one of two categories. Either, the device generates positive pressure pulses or increases of pressure within the drill string over a defined basal level, or generates negative pressure pulses or decreases of the pressure for the drill string. Le Peuvedic, et al., "Hydraulic Controlled Device For Modulating the Mud", U.S. Pat. No. 3,737,843, is an example of a positive pulsing mud valve. A needle valve is mechanically coupled to a piston motor in Le Peuvedic. The needle valve acts against a fixed seat. The piston motor in turn receives the continuous flow of control fluid. Information is transmitted to the surface in the form of rapid pressure variations ranging from 5 to 30 bars and succeeding one another at intervals of 1-30 seconds. Each pressure pulse is generated by reversing an electric current passing through a solenoid coil which is coupled to the needle valve.
Arps, "Earth Well Borehole and Logging System", U.S. Pat. No. 2,925,251, is also directed to a positive pulse telemetry system. A current pulse is applied to a magnet of a control valve. Activation of the valve by the magnet causes an increase in pressure in a cavity outside the valve body. The valve body fluxes and propagates a pressurized signal into the mud stream. Thus, Arps shows a positive pressure pulse system which is operated by several valves.
Spindler, "Pilot Operated Mud Pulse Valve", U.S. Pat. No. 3,9588,217, is also directed to a positive mud pulse telemetry system. In the absence of the generation of mud pulses, the mud flows through an upstream collar through a valve into an annular passage. Mud flows through interior passageways parallel to the main mud stream past a pilot valve seat and through a number of passages to rejoin the main mud flow. Therefore, by actuation of the bypass of valving, positive mud pulses can be generated in the main flow.
Gearhurt, et al., "Downhole Signaling System", U.S. Pat. No. 3,964,556, is yet another example of a positive mud pulse telemetric system.
Westlake, et al., "Method of and Apparatus for Telemetry Information From a Point in a Well Borehole to the Earth's Surface," U.S. Pat. No. 4,780,620, shows a negative mud pulse system. A motor driven valve is open inresponse to binary signals generated by an package downhole. Upon opening a portion of the mud flow is allowed to escape from the drill string to the annulus between the drill string and borehole.
However, each of the prior art systems are binary mud pulse systems capable of generating a mud pulse either above or below a basal pressure rate which is characterized as a normal pressure. However, downhole telemetering systems have to be able to transmit a huge volume of downhole information regarding the nature of the drilled formations, directional information, and conditions of the borehole.
Therefore, what is needed is a system and method for selectively providing a pulsing telemetry best suited for the application on hand and further capable of providing telemetric information with a signal protocol more efficient than prior art binary systems, which will provide significantly increased data rates.